The present invention relates to an image reading device that adjusts for weakening of scanner elements over time. The present invention also relates to a method for formulating a level of image signals, and to an electronic board which performs the image signal level formulating method and is provided with the image reading device.
There has been known an image reading device for use in an electronic whiteboard. The image reading device includes a scanner and a comparator. The scanner includes a plurality of sensor elements aligned in a single row for optically reading the image on the board. The optical sensors pick up an image and output an image signal accordingly. The comparator compares the image signal from the sensor elements with a predetermined threshold value to produce binary data of the image signal. That is, the comparator judges a pixel to be white or black based on the threshold value.
There is variance in the sensitivity of the optical sensors of the scanner. To adjust for this variance, a reference image signal level curve V1 and a threshold level curve V0 shown in FIG. 1(a) are prepared before shipping the image reading device from the factory, and prestored in a memory unit of the image reading device as the characteristic of the image reading device.
That is, at the factory the scanner of the image reading device is used to scan a predetermined white color reference member. The obtained image signal is corrected to match its peak value with a maximum value Vmax, which is the maximum value established for the readable range of the image reading device. The corrected image signal is stored in the memory unit as the reference image signal level curve V1. Then, the threshold level curve V0, which is used to convert a signal of a read out image into binary image data as described above, is determined based on the image signal level curve V1. That is, the image signal level curve V1 is multiplied times a predetermined coefficient to produce the threshold level curve V0. The threshold level curve V0 is also stored in the memory unit. Later, when the image reading device is actually used to read images, the actual image signal level curve is compared with the threshold level curve V0 to produce an output of binary image data.
However, the scanner of this conventional image reading device can degrade over time and/or for other reasons, so that the scanner outputs an image signal level smaller than the image signal level measured at the factory. However, because the weaker image signal is compared with the same threshold level curve V0 to produce binary data, more pixels will be judged as being black, resulting in an overall darkening of images printed out based on the binary data.
Therefore, adjustments must be made before the scanner is used to correctly read images, in order to increase the image signal level as appropriate for the threshold level curve V0. For the adjustment, the scanner is used to scan a white reference surface disposed at the image reading position of the device, to read an image signal level curve V2 shown in FIG. 1(b). Then, the image signal level is adjusted so that a maximum value V2max of the image signal level curve V2 matches the above-described maximum value Vmax, to produce a corrected image level curve V2corrected as shown in FIG. 1(c). After the adjustment of the image signal level, the image reading device is actually used to read images, and the actual image signal level curve is compared with the already stored threshold level curve V0 to produce an output of binary image data.
However, the white reference surface disposed at the image reading position of the scanner can be stained or soiled. This is particularly common with electronic whiteboard that are written on using a felt-tipped marker, because the ink from the felt-tipped marker is often not completely erased from the white reference surface. In this case, the stained area will affect the image signal level curve. As shown in FIG. 1(b) the entire shape of the image signal level curve V2 will be different from the entire shape of the reference image signal level curve V1. (In FIG. 1(b), a curve V1′ is an imaginary curve provided that no stain portion is provided in the whiteboard but the signal level are entirely weakened as a result of degradation.) That is, the signal level are lowered at the stained area of the whiteboard.
With this state, if the curve V2 is modified so that the maximum value V2max of the image signal level curve V2 matches the above-described maximum value Vmax as shown in FIG. 1(c), a broken line curve V3 in FIG. 1(c) which corresponds to the broken line curve V1′ exceeds the maximum readable range Vmax.
In the factory before shipping, assuming that there is a black color area in a whiteboard, and a black dot is shown as an image signal level. This black dot is located below the threshold level curve V0 as shown in FIG. 1(a), so that the determination falls within a black color range. On the other hand, after the above-described adjustment, the black dot is positioned above the threshold level curve V0 as shown in FIG. 1(c) due to excessive adjustment of the image signal level, so that the determination falls within a white color range. In summary, due to the excessive adjustment, the read image data may produce an entirely white or pale image in comparison with the actual image drawn on the whiteboard.